1. Field of the Invention
The present invention relates to an image-forming device and a scanning unit, and particularly to an image-forming device and a scanning unit including a polygon mirror or other deflecting means for scanning light beams, the polygon mirror having a plurality of reflecting surfaces on which a plurality of light beams is obliquely incident from different positions in a subscanning direction.
2. Description of the Related Art
In a laser printer or other electrophotographic image-forming device, a light source such as a semiconductor laser emits a light beam that is deflected by a polygon mirror or other deflecting means onto a photosensitive member, the surface of which has a uniform charge, so as to form an electrostatic latent image on the surface of the photosensitive member by scanning the light beam thereon. The latent image is subsequently developed into a visible image with toner, and the toner image is transferred onto a recording medium, such as a sheet of paper, to form an image on the recording medium.
Conventional image-forming devices form color images by superimposing toner images in the colors cyan (C), magenta (M), yellow (Y), and black (K) Color image-forming devices that employ the tandem system are provided with a plurality of image-carrying members (photosensitive drums or the like), one for each color. The surface of each image-carrying member is exposed by a light beam according to image data corresponding to each color image, forming a latent image that is subsequently developed with toner. The toner images of each color are then transferred sequentially and superimposed over one another on an intermediate transfer medium or a paper or other recording medium, thereby forming a color image.
The laser printer is one conventional example of an image-forming device that scans and exposes an image-carrying member. In this exposure process, a light-emitting unit of the laser printer, such as a semiconductor laser, emits a laser beam. When the laser beam, having divergent rays, passes through a collimator lens, the collimator lens converts the light beam into substantially parallel rays. In order to perform face tangle error correction, a cylindrical lens is provided to converge the light rays in a subscanning direction, guiding the light onto a polygon mirror that is rotating at a high speed. The light beam is reflected off the polygon mirror, passes through an optical scanning system having scanning lenses, and is scanned over the surface of the image-carrying member.
Japanese Patent Application Publication No. 2004-21133 describes a conventional optical scanning device provided with a plurality of image-carrying members, as in the tandem method described above (or a single image-carrying member scanned at different positions), that employs a single polygon mirror to deflect a plurality of light beams emitted from light-emitting means corresponding to each color. In this technology, the light-emitting means are disposed at different positions in the subscanning direction for emitting a plurality of light beams that are obliquely incident on reflecting surfaces of the polygon mirror. Optical elements such as reflecting members corresponding to each of the light beams reflected off the reflecting surfaces of the polygon mirror are provided for forming restricted optical paths for guiding the light beams to the corresponding image-carrying members in order to expose the surfaces of the image-carrying members.
However, there has been much demand in recent years to produce more compact image-forming devices, such as the tandem image-forming devices described above for forming color images using a plurality of image-carrying members (or a single, large image-carrying member). In the technology of Japanese Patent Application Publication No. 2004-21133 described above, a plurality of laser beams are irradiated obliquely onto a single reflecting surface of a polygon mirror and deflected therefrom, and the plurality of laser beams are appropriately reflected off corresponding optical elements such as reflecting mirrors so as to be guided to corresponding image-carrying members. Since a relatively large distance must be maintained between the reflecting surface of the polygon mirror and the reflecting mirrors or other optical elements, this technology does not contribute to the production of a more compact device.